Estimating the Causal Effects of Social Networks on Health Behaviors Individuals in modern societies are socially connected in a multitude of ways. Individuals use their social networks to receive and send information as well as establish, update, and enforce social norms of behavior. Both information acquisition and the impact of social norms within social networks could have large effects on the health behaviors of individuals, especially adolescents, who are particularly responsive to peer pressure. This heightening of peer influence also takes place during the developmental stage when many of the most costly health outcomes and behaviors are initiated. This confluence of events during adolescence sets the stage for a critical period of individual decision-making with long-term consequences and also provides a unique window of opportunity for health intervention. While much recent research has shown robust and important correlations in the health behaviors of socially connected individuals, whether the correlations are causal is still largely unknown for many health behaviors. This application aims to estimate the potential causal relationships and also illuminate potential mechanisms that may be utilized in future interventions. We propose to undertake the development and demonstration of new empirical techniques to evaluate the importance of social networks in the initiation, diffusion or cessation of healthy and unhealthy behaviors. This project will utilize social network data from the National Longitudinal Study of Adolescent Health (Add Health), which is the largest study of adolescent friendship networks ever conducted and is nationally representative. Using these data, we will address current gaps in the research knowledge base by pursuing the following aims: (1) We will develop and demonstrate new quantitative methods to estimate the causal effects of friend influences on health behaviors during adolescence, including tobacco and alcohol use, weight and other outcomes; (2) We will develop and demonstrate new quantitative techniques to examine the influence of peers on the timing of initiation and duration of unhealthy behaviors; (3) We will explore policy relevant structural elements of friendship networks (distance of social ties, measures of importance in a social network) in order to examine how to target resources to maximize the health benefits of an intervention. We focus on adolescence due to its importance in determining initiation into a host of unhealthy behaviors as well as the importance of peer influence during this development stage. However, the methods we develop are general enough to apply to other social networks. Findings from our investigations will increase understanding of the role of social networks in health behavior determination as well as enhance our ability to propose specific policies targeted to attain the largest benefits of intervention.